1. Field of the Invention
The present invention relates to an active low-pass filter which consumes less current and assures excellent distortion characteristic.
2. Description of the Related Art
FIG. 3 is a circuit diagram illustrating an active low-pass filter of the related art. As illustrated in FIG. 3, the active low-pass filter of the related art comprises of a common-emitter amplifying circuit 31 of the first stage and a common-base amplifying circuits 32, 33 of the second and third stages, where a feedback resistor 34 is connected, as a negative feedback circuit, between an output end of the common-base amplifying circuit 33 of the third stage as the common-base amplifying circuit of the final stage and an input end of the common-emitter amplifying circuit 31 of the first stage, a first capacitor 35 for setting the cutoff frequency is connected between the connecting point of the common-emitter amplifying circuit 31 and common-base amplifying circuit 32 and the ground, and a second capacitor 36 for setting the cut-off frequency is also connected between the connecting point of the common-base amplifying circuit 32 of the next stage and the common-base amplifying circuit 33 of the third stage and the ground.
The collector of transistor Q1 and emitter of transistor Q2 are DC-connected via a resistor 37 so that a common collector current flows into the transistors Q1, Q2, Q3 of each amplifying circuit 31, 32, 33 and the collector of transistor Q2 and the emitter of transistor Q3 are also DC-connected via a resistor 38. Resistors 37, 38 are provided to reduce distortion generated in the common-base amplifying circuits 32, 33 having a low input impedance.
The power supply voltage is applied, via a load resistor 39, to the collector of transistor Q3 as the output end of the common-base amplifying circuit 33, while the emitter of transistor Q1 of the common-emitter amplifying circuit 31 is grounded through a resistor 40.
The feedback resistor 34 sets amount of feedback current and also gives abias voltage to the base of transistor Q1 of the common-emitter amplifying circuit 31 of the initial stage. Therefore, this feedback resistor 34 is connected between the base of transistor Q1 as the input end of the common-emitter amplifying circuit 31 of the initial stage and the collector of transistor Q3 of the common-base amplifying circuit 33 of the final stage and a bias resistor 41 is also provided between the base of transistor Q1 and the ground. Thereby, a bias voltage is applied to the base of transistor Q1. Moreover, the bias voltage is also applied to the bases of transistors Q2, Q3 through the bias resistors 42, 43, 44.
In the structure explained above, a high frequency signal input from a signal input terminal 45 is amplified by the common-emitter amplifying circuit 31 through a coupling capacitor 46 and the high frequency element of this signal is cut by the first capacitor 35. The high frequency signal is further amplified by the common-base amplifying circuit 32 through a resistor 37 and the high frequency element of this signal is then cut by the second capacitor 36 and is then input to the common-base amplifying circuit 33 of the final stage. The high frequency signal amplified by the common-base amplifying circuit 33 of the final stage is output from a signal output terminal 49 through a capacitor 47, for impedance matching, and a coupling capacitor 48.
In the active low-pass filter of the related art explained above, since a bias voltage is supplied to the base of the transistor in the common-emitter amplifying circuit of the initial stage through a feedback resistor connected to the collector of the transistor in the common-base amplifying circuit of the final stage and to the base of the transistor in the common-emitter amplifying circuit of the initial stage, a current flowing through the feedback resistor has been wasted. Moreover, since abias voltage is applied to the base of the transistor in the common-emitter amplifying circuit of the initial stage, a resistor is provided between the base and the ground, thereby resulting in loss of input signal.